Cycling around the geology and geomorphology of Midlothian Pictures from www.gorebridge.org.uk Cycling is probably the best way of appreciating terrain. In this 52-km (32-mile) long trip, mostly on quiet roads and cycle paths, we make the link between terrain, underlying geology and the processes that shape landscapes. The following topics are covered: Carboniferous volcanisms, Carboniferous sediments and the coal industry, the Pentland fault, shaping of the landscape by glaciers and rivers and the influence of contrasts in rock resistance to erosion on landscape morphology. The event takes a whole day and is ideally for people with minimum experience of cycling relatively long distances on hilly terrain. The trip essentially explores the Esk River basin and includes a ~1:30 hours visit of the National Mining Museum in Newtongrange (Scottish tourist board five-star attraction): http://nationalminingmuseum.com/ A map of the itinerary is provided (using OS Map 66 -Edinburgh- as the base map), as well as some information about the geology and geomorphology of the area, including a lovely leaflet about the geology and scenery of the Esk Valley commissioned by Lothian and Borders GeoConservation (visit the Esk Valley Trust website: http://www.eskvalleytrust.org/ ). Ideal equipment includes: good bike with functioning front and back lights, helmet and high visibility jacket. You may want to bring food and drinks, as well as a lock for your bike and some shoes to walk on a potentially muddy path (the trip includes short walks). IMPORTANT: bring warm clothes for the visit of the museum!!! The visit is essentially outdoor or within the mine and it can get VERY cold. Note that Lothian Buses serve many of the towns visited (e.g., service 29 to Newtongrange and Gorebridge, service 40 to Roslin, service 37 to Loanhead, etc…) Dr Mikaël Attal School of GeoSciences, University of Edinburgh [email protected] Edinburgh, August 2015 0 Cycling around the geology and geomorphology of Midlothian Route map (base map = OS Map 66) Stars with arrows indicate locations where the route leaves National Cycle Route 1 9 1 3 2 Inset stop 3 3 8 7 4 5 6 1 km Approximate distances: 1 to 4 = 21 km (13 miles) 4 to 7 = 19 km (12 miles) 7 to 9 = 12 km (7.5 miles) Cycling around the geology and geomorphology of Midlothian Before starting, make yourself familiar with the geology and geomorphology of the area. The Esk Valley leaflet (page 3-6) gives an excellent introduction to the geological history of the area: the rocks exposed are igneous and sedimentary rocks, Ordovician to Carboniferous in age (380 to 300 million year old, see Geological timescale p. 4 and Scotland’s geological history with geological map p. 5-6). The landscape we see today is essentially the result of shaping by glaciers during the Quaternary; glaciers were covering most of Scotland during the Last Glacial Maximum, which ended only ~20,000 years ago (see Recent History p. 4 and Slipping Slopes p. 7). The relief of the area is shown on page 8 and 9: high relief is restricted to the Pentland Hills, with the Esk basin contained in a wide trough immediately to the SE of the hills. When geology is superimposed onto the topography (p. 10-12), the importance of the geology appears clearly, with the Pentland Hills being made of volcanic rocks (orange, purple) that are more resistant to erosion than the adjacent sedimentary rocks (grey, yellow, blue). The sharp contact between the two types of rocks, the Pentland Fault, appears as a sharp contact in the topography (hills rise abruptly at the fault). When glaciers were moving over the area, they carved more easily into the sedimentary rocks than into the volcanic rocks, leaving the volcanic rocks in relief (with for example: Arthur’s Seat, Corstorphine Hill, the Pentland Hills, North Berwick Law, Bass Rock, to cite only a few). Note that the orientation of the geological structures seems to have influenced the flow direction of the ice: in the area, most geological structures are oriented NE-SW (including the Pentland Hills) and the streamlined NE-SW features in the Esk Basin visible on the hillshade map of the area (p. 9) are evidence that the ice was moving in the same direction (it was actually moving to the NE towards the Firth of Forth, see p. 7); you will look at these features more closely during the trip. Finally, it is interesting to note that the Esk basin sits in a syncline, that is, a fold where the youngest rocks are found in the middle; the cross-section on page 13 will give you an idea of the structure of the different rock units under the surface (the location of the cross-section is shown on the map p. 10). Enjoy the ride! Description of sites indicated on route map: 1- Starting point on Holyrood Park Road: make yourself familiar with the geological and geomorphological history described in this leaflet and admire the volcanics that make up Arthur’s Seat. This is a Carboniferous volcano (~300-350 million year old) and the vertical columns of Salisbury Crags are the result of the cooling of basalt (similar to the famous Giant’s Causeway in Ireland). Join Cycle Route 1 through the Innocent Railway Tunnel (most of the cycle paths you will use today were railways in the past). 2- Stop at the Queen Margaret campus by Musselburgh, before going over the railway, and look back at the prominent Arthur’s Seat. Prepare to descend into the Esk Valley. 3- Leave the main road and enter an area which is part of Dalkeith Country Park (see inset on map p.1). Down by the river, under the A68, you will find a spectacular coal seam within the Carboniferous fluvial sandstones (p. 6). DO NOT HAMMER OR EVEN PICK AT ROCKS AT THE EXPOSURE. The exposure is fragile and there are enough fragments underneath. Note also that there are a lot of horses in this part of the estate and that the horse people don’t like cyclists so much (they scare horses): make sure you move slowly and stop if approached by horses. DO NOT CYCLE ON THE PATH ALONG THE RIVER: leave your bike at the bridge; ideally, leave your bike at the entrance of the estate (by the main road). 1 4- Visit the National Mining Museum in Newtongrange. Plan around 1:30 hour for the visit. You will be guided through the mine working, its history and regional significance by ex- miners. Make sure you have very warm clothes, even in summer. And book in advance if you have a big group. This is usually a good place to have lunch and there is a café, restaurant and toilets in the museum if needed. 5- Stop by the school layby in Gorebridge for a great view over the Esk basin. Locate yourself on the hillshade map p. 9. From this point you can see the Pentland Hills and Arthur’s Seat in the distance. In front of you, the Esk basin which appears so flat on the map shows quite a bit of relief; a lot of this relief is made of the streamlined features we mentioned earlier. From 5 to 6, you will cycle across these features and across a series of rivers that generally flow towards the NE, towards the Firth of Forth (including the South Esk and the North Esk which join in Dalkeith to form the Esk River). On your way to 6, your legs will make you notice that the area is not flat! Note the smooth appearance of the landscape (rolling hills) and the lack of rock exposures: the landscape is almost entirely blanketed with soft sediment which has been produced, transported and deformed at the bottom of a glacier! Many of the streamlined features are mounds of soft sediment shaped by the movement of the ice as it was moving towards the NE. On your way, note also the deep river valleys, with narrow gorges tens of meters deep. If a 20 m deep gorge had been formed by a river after deglaciation, this would imply erosion rates in excess of 1 mm/yr (to form a 20 m deep gorge in less than 20,000 years). Such erosion rates are typically found in tectonically active places such as the Himalayas, Taiwan or the Alps, so they are quite surprising here in Scotland! Potential explanations include: (1) erosion is essentially in soft sediment so it can be rapid; (2) during deglaciation, rivers may have been much more powerful than now (with higher water discharge) so they could have eroded their bedrock very rapidly; (3) maybe these rivers were sub-glacial streams: the gorges may have been (at least partly) carved by the rivers under the glacier before deglaciation… 6- Nice location for a last panorama over the Pentland Hills to the west and the Esk basin and its hills of soft sediment to the east. Prepare to descend into Roslin Glen (North Esk River). 7- Break in Roslin, with its nice wee pubs and a well-known chapel (since the Da Vinci Code). 8- If time permits, you may want to look at the soft glacial sediment at Hewan Bank. The glacial sediment (or till) is very often covered in vegetation. At Hewan Bank, the meandering river has cut into the hillslope and caused a landslip, offering a rare opportunity to see what the till is made of (see p. 7). To access the site, take a small path to the right BEFORE you pass over a long iron bridge over a deep valley (see map).